1. Field of the Invention
The present invention relates to a method and a device for fitting a surface to a point group, a modeling device and a computer program. The present invention is useful in the field of computer graphics such as generation of a three-dimensional model, etc.
2. Description of the Prior Art
In recent years, the three-dimensional CG (three-dimensional computer graphics) technique is often used in the fields of movie, game and so on. In the three-dimensional CG, a three-dimensional model, light source, etc. are located and moved in a virtual three-dimensional space; therefore, the three-dimensional CG is high in the degree of freedom of expression.
Conventionally, a non-contact three-dimensional measurement device employing the light-section method has been put into practical use. Three-dimensional data of an object can be relatively easily obtained by using such non-contact three-dimensional measurement device. However, various problems have been detected in using the three-dimensional data obtained by means of the non-contact three-dimensional measurement device as it is for three-dimensional CG. For example, it is necessary to reduce the amount of data obtained by the measurement by a process of thinning the data, which is complicated and time-consuming.
To solve the above problems, there has been proposed a method wherein a standard model is prepared for an object and the standard model is modified according to three-dimensional data of the object obtained by an actual measurement of the object (Japanese Unexamined Patent Publication No. 5-81377).
In the above-mentioned method, three-dimensional form information of the three-dimensional data obtained by the measurement, i.e., a point group in the three-dimensional space, is used as an object for fitting, and a surface of the standard model is fitted on the three-dimensional point group. According to the method, it is possible to obtain three-dimensional data or a three-dimensional model free from defects even in the case where the three-dimensional data originally had defects.
However, a satisfactory result is not yet achieved by the above method, since, in modifying the surface simply according to the point group obtained by the measurement, the surface is unavoidably fitted to some false points included in the point group, which are low in reliability.
Specifically, as shown in FIG. 19A, in the case where the points P3 to P6 among the six points P1 to P6 are low in reliability, the surface S is modified according to all the points P1 to P6. The surface Sa obtained by the modification is not always satisfactory or proper.
Further, as shown in FIG. 19B, in the case where points P3 to P6, which are low in reliability, are ignored and reliable points P1 and P2 are used, the modification itself cannot be fully achieved with failing to reflect and maintain a form of an object represented by the point group. The surface Sb obtained by the incomplete modification is hardly a proper representation of the form of the object.
The three-dimensional model obtained by the above-mentioned method is frequently utilized in the field of animation. In the field of animation, not only a skin model which is external and visible for human eyes, but also a skeleton model or a muscle model (muscle information) which moves the skin model as lying thereunder is utilized.
However, since the three-dimensional data obtained by measurement of an object corresponds to the skin model, it is impossible to generate the skeleton model or the like from the three-dimensional data as it is.
Further, in the case of adopting modification information of a skin model as it is for generating a skeleton model based on the three-dimensional data of the object, it is possible that the skeleton model is improperly modified due to the difference between the skin model and the skeleton model.
Although a general form of an object can be reproduced according to the above method, it is difficult to reproduce topical characteristics of a form of the object.
For example, in the case of making a three-dimensional model of a human head according to the above method, although general form of the resulting three-dimensional model conforms to an object for measurement, i.e., the human head, parts that influences much for expression of the face such as eyes, mouth and the like do not satisfactorily conform to those of the object. Thus, there have been problems relating to the representation of delicate expressions of human.